The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for constant voltage mode and constant current mode. Merely by way of example, the invention has been applied to a flyback power converter with primary-side sensing and regulation. But it would be recognized that the invention has a much broader range of applicability.
Flyback power converters have been used extensively for their simple structures and low costs in low power applications. But in traditional flyback converters, the output voltage regulation often is performed with secondary-side feedback, using an isolated arrangement of TL431 and an opto-coupler. In addition to increasing the system cost, the voltage drop due to the cable loss usually is difficult to compensate.
FIG. 1 is a simplified conventional diagram for a switch-mode flyback power conversion system with secondary-side control. As shown in FIG. 1, a PWM controller 110 is used to control and drive a power MOSFET M1. The power MOSFET M1 is turned on and off to control the power delivered to the load on the secondary side. Consequently, the constant output voltage (CV) mode and the constant output current (CC) mode may be achieved by the secondary-side regulation.
FIG. 2 is a simplified conventional diagram showing characteristics of output voltage and output current of a flyback power conversion system. As shown in FIG. 2, if the output current Io is in the range of from zero to Imax, the system operates in the constant voltage (CV) mode. In the CV mode, the output voltage Vo is equal to Vmax. Alternatively, if the output voltage is below Vmax, the system operates in the constant current (CC) mode. In the CC mode, the output current Io is equal to Imax. For example, if the output terminal of the system is connected to a discharged battery, the system operates in the CC mode.
To reduce cost and size of the switch-mode flyback power converter and to also improve its efficiency, the power converter with primary-side regulation has become more and more popular. With the primary-side regulation, the output voltage is sensed by detecting the voltage of an auxiliary winding that is tightly coupled to the secondary winding. Since the voltage of the auxiliary winding images the output voltage that is associated with the secondary winding, the voltage sensed in the auxiliary winding can be utilized to regulate the secondary-side output voltage. The expensive parts of TL431 and opto-coupler usually are not needed, so the cost and size can be reduced. Additionally, using sensed information of the output voltage, the output current can be regulated based on internal computation of the controller. Therefore the sensing resistor for output current often is not needed, so the overall conversion efficiency can be improved.
FIG. 3 is a simplified conventional diagram for a switch-mode flyback power conversion system with primary-side sensing and regulation. FIG. 4 is another simplified conventional diagram for a switch-mode flyback power conversion system with primary-side regulation.
As shown, the output voltage Vout is mapped to the DC voltage VINV at the node INV, and is therefore regulated through the regulation of VINV.
With primary-side regulation, the relationship of VINV and Vout can be expressed as:
                              V          INV                =                                                            n                ·                                  R                  2                                                                              R                  1                                +                                  R                  2                                                      ·                          (                                                V                  out                                +                                  V                                      D                    ⁢                                                                                  ⁢                    2                                                              )                                -                                                    R                ⁢                                                                  ⁢                2                                                              R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  2                                                      ⁢                          V                              D                ⁢                                                                  ⁢                1                                                                        (        1        )            
where n is the ratio of auxiliary-winding turns to secondary-winding turns. Additionally, VD1 and VD2 are the forward diode drop voltages.
Setting
      k    =                            R          1                +                  R          2                            n        ·                  R          2                      ,Vout is therefore given by:
                              V          out                =                              k            ·                          V              INV                                +                                    1              n                        ⁢                          V                              D                ⁢                                                                  ⁢                1                                              -                      V                          D              ⁢                                                          ⁢              2                                                          (        2        )            
The output voltage is regulated through the regulation of the voltage for the auxiliary winding. For example, the sensed voltage, VINV, is compared with a predetermined voltage level, VREF. The difference between VINV and VREF is associated with an error signal, which is amplified by an error amplifier. Based at least in part on the amplified error signal, a PWM/PFM signal is generated.
The PWM/PFM signal controls turning on/off of a power switch and thus controls the power delivered to the secondary side. As a result, the difference between VINV and VREF becomes smaller and smaller, and eventually VINV becomes equal to VREF. Since VINV is the image of the output voltage Vout, the output voltage Vout can be linearly dependent on VINV and thus VREF, if certain conditions are satisfied.
Specifically, as shown below, the output voltage Vout linearly depends on VREF if the forward voltage across diodes D1 and D2 are constant.
                              V          out                =                              k            ·                          V              REF                                +                                    1              n                        ⁢                          V                              D                ⁢                                                                  ⁢                1                                              -                      V                          D              ⁢                                                          ⁢              2                                                          (        3        )            
But the forward voltage of a diode often depends on the current that flows through the diode. Hence the forward voltage of D2 changes if the load current changes. The forward voltage of D1 is almost constant since the current flowing through D1 does not change even if the output load current changes.
Therefore, the control scheme as described above often has poor regulation for output voltage due to the change in the forward voltage of the diode D2. Moreover, the fact that the output current depends on the inductance of the primary windings often results in large variations in the output current which usually cannot be compensated in the mass production.\
Hence it is highly desirable to improve techniques for output voltage regulation and output current control, such as primary-winding inductance compensation, is highly desirable.